NZ272570A - Preparing half tone images: creating half tone dots with inwardly curved edges, and aligning primary colour screens at odd multiples of 45 degrees to each other - Google Patents

Description

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NEW ZEALAND PATENTS ACT, 1953 Divided from No. 239389/ Filed 13 August 1992 Cognated from No.: No.: 240S Dated: 13 Au| lovember 1991 v' )atear~-W3e£B^beiLl991 Dated: 1 May 195 COMPLETE SPECIFICATION HALFTONE DOT PATTERNS . t <*'« / ,t * A v ± V 13 JUL 1995 .«.c We, MEGADOT SYSTEMS LIMITED, a New Zealand company of 60 Kent Terrace, Wellington, New Zealand, hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: 27257 HALFTONE DOT PATTERNS BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to printing processes and in particular to dot patterns used when preparing halftone images. These patterns include shapes which reduce the perceptibility of certain moir§ effects and various tone jumps which are often seen.

DESCRIPTION OF THE PRIOR ART In most printing processes it is only possible to apply a single tone of each available ink colour to the print media. Tone variation is then achieved by breaking up each image into fine dots of varying size on a halftone screen grid. Colour variation is normally achieved by superimposing screens of the primary colours cyan, magenta and yellow, plus black for definition. Ideally human vision integrates the dots over a well prepared image into an accurate impression of the original scene. A final image will almost always include a number of compromises between practical limitations and defects in the printing process, and what can actually be perceived by the human eye and brain.

Round dots at 45° to vertical are least perceptible for a given spacing or screen ruling (or dot frequency), and single color images are conventionally printed in this manner. Other dot shapes or combinations of shapes such as square and "elliptical" (diamond shape) are sometimes used, but all have generally straight or outwardly curved edges around the full length of their 272570 perimeters. The dots are usually created in square cells forming rows spaced at between about 30/cm for newsprint and about 60/cm for higher quality images. In light tones the dots remain distinct on a light background provided by the print medium, but merge in darker tones which ther. appear as light dots on a dark background. The printed and non-printed areas of an image therefore appear to reverse from dots to background and background to dots respectively as tone darkens.

When printing colour images undesirable moird effects in the form of large and small scale patterns are often seen due to periodic alignment of the dots as a whole and of their edges. The large scale patterns are typically bands which intersect to form squares on the order of tens or more dots along each side. This effect is largely removed by suitable relative rotation of the colour screens such as by cyan 15°, magenta 45°, yellow 90° and black 75° anticlockwise from horizontal. Complex mathematical procedures are often used to establish suitable angles. Placingihe screens without relative rotation or offset can cause colour shifts in an image where colours of differing opacity, particularly black and yellow, overlap consistently over a large region. Subtle colour shifts may occur in any case due to mis-registration of the screens during their superposition. The small scale patterns of moird effects are typically rosettes on the order of a few dots width, which cause perceptible speckling of otherwise uniformly coloured areas. This effect has proved more difficult to remove.

A further problem often arises in printing halftone images, known as dot gain, amid tones where adjacent dots are so sufficiently large as to become linked by imprecision in their reproduction. For example in lithography, which includes offset printing, a greasy ink is confined to printing areas of an image plate by dampening the surrounding non-printing areas with water. Unfortunately surface 272570 tension at the ink/water interfaces can cause or enhance bridging between closely spaced printing areas creating sudden tone jumps. Ink absorption on poor quality print paper can also lead to bridging. Round and square dots formed in square cells naturally meet their nearest neighbours at 78% and 50% printing area densities respectively. Dot gain causes bridging at slightly lower densities and enhances bridging at slightly higher densities creating discontinuities in regions of an intended smoothly varying tone. This effect is also difficult to remove completely.

Preparation of halftone images is largely carried out using computer controlled devices such as scanners and imagesetters. A photograph or other artwork to be reproduced is scanned and the original scene is stored in electronic memory or output directly. The images can be manipulated and/or combined with text before a printing medium such as a film or plate is produced. It is normally only in the final output stages that an image is converted to halftone dot screens. The manipulations are complex software operations which may be varied to suit particular images. Similar software is used in other electronic printing and imagesetting processes such as desktop publishing. Precise control of the dot patterns is necessary in preparing acceptable images, and the computations required for high quality images are often extensive and time consuming. For example, software methods for reducing moir6 effects are disclosed in US 4,084,183, (congruent screens), US 4,894,726 (quasi periodic screens), EP 370271 (elongated conventional dots), WO 90/10991 (rectilinear screen transposition) and WO 90/06034 (pseudo random variation of dot shapes). Some images are still prepared using conventional photomechanical equipment such as contact screens. Good summaries of known dot patterns, their various problems and moir6 effects are to be found in Colour Screening Technology; A Tutorial on 27 2 57 0 the Basic Issues, The Seybold Report on Desktop Publishing, Vol 6, No. 2, October 1991, Seybold Publications Inc., PA, USA, and Desktop To Press, Issue 9, February 1992, Peter Fink Communications Inc., CA, USA.

SUMMARY OF THE INVENTION It Is an object of the present invention to provide an alternative dot pattern which can be used to reduce rosette moir6, dot gain and tone jump effects in halftone images.

A halftone image having a dot pattern according to the invention is created by printing dots having inwardly curved edges. In light tones the dots may resemble pin cushions having pronounced cusps. In dark tones the dots are effectively merged to create non-printing dots which may be oval shaped. As tone varies from light to dark, the pin cushions are increasingly elongated along one direction and meet their nearest neighbours in two distinct stages, first in the direction of elongation and then in a direction substantially perpendicular to it. The dot screen is typically formed from a square cell so there are typically four nearest neighbours symmetrically placed at equal distances. As tone continues to darken after the dots have joined, the elongation is gradually decreased so that the oval shaped non-printing dots approach round dots. The pattern may also be considered overall as a reversed elliptical dot pattern in which the ellipses vary from round to a maximum ellipticity and back to round across the full range of tones. When a number of screens are superimposed the inwardly curved edges of the printing areas do not align so readily to form perceptible rosettes as do conventional dots. Further, the elongation may be varied so that dot gain occurs in tones where its perceptibility also is minimised.

N-Z. ; '.V.: ' OFFICE 2 6 JUL 1936 272 5 7 0 At the present time as this specification is prepared, the optimum uses and ramifications of the invention have not been fully explored. In preparing colored images it has been found that the primary color and black screens are best placed so that their dot patterns lie at approximately 45° separations from each other. For example cyan 45°, magenta 135°, yellow 90° and black 0°. Screens which are separated by 90° may generally be regarded alternatively as being at 0° so that in this example there are essentially only two angles for computation of dot patterns, 0° and 45°. This represents a considerable computational simplification over conventional screen angles.

A method and apparatus relating to the dot elongation has been claimed in NZ 239389/240636/240979/242583 from which the present specification has been divided. A method and apparatus relating to 45° screen separation is claimed in the present specification.

The cyan and magenta screens will normally have a deliberate offset or mis-registration from each other to avoid possible color shifts. Yellow and black screens may also be offset from each other, and may be printed with little or no elongation of the dots. The yellow and/or black screen rulings may also be increased and/or decreased relative to cyan and magenta. Final determination of these possibilities awaits full software implementation of the invention and will depend on particular images.

BRIEF DESCRIPTION OF THE DRAWINGS Examples of the invention will be described with reference to the drawings of which: n.Z. r 2 6 JUL 1996 r f 272570 Figures 1a, 1b and 1c show coarse conventional round, square and elliptical dot patterns respectively, varying uniformly between light and dark tones; Figure 2 is an illuminated globe image using a conventional variable round dot pattern; Figure 3 shows reversal of a coarse conventional round dot pattern to produce printing areas having inwardly curved edges; Figures 4a and 4b show reversal of a variable oval dot pattern in which the dots vary from round to oval and back to round according to the invention; Figures 5a, 5b and 5c show example variable oval dot outlines superimposed upon common centres according to the invention; Figures 6a and 6b show respectively two conventional round dot screens overlapping at 30° and two reversed dot screens according to the invention overlapping at 90°; Figures 7a to 71 show example dot patterns for various screen combinations according to the invention; Figures 8a to 8i show corresponding conventional round dot patterns for contrast with figures 7a to 7i; Figure 9 is a flowchart indicating a general process in which the invention may be implemented; and Figure 10 is a schematic diagram of hardware on which the invention may be implemented.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to these figures it will be seen that the dot patterns are magnified about 10 to 100 times in black and white for purposes of their description. The n.z. patent office 7 - MAR 1996 RECEIVED 272570 visual effects resulting from integration by the human eye over a halftone image at normal scales are not evident but should be appreciated by a skilled reader. Particularly the improvements to be obtained in colour images by inwards curvature of printing dot edges in light to middle tones and to be obtained in single color images by dot elongation will be appreciated.

The various forms of computer hardware and software used to implement dot patterns according to the invention will also be known to the skilled reader, or at least will be available for consideration through the references given above. For example, a range of desktop publishing and high end scanner equipment and software is available through suppliers such as Adobe, Agfa, Crosfield, Linotype-Hell and Scitex. As new patterns are developed the hardware and/or software may be correspondingly upgraded. A typical process of preparing a halftone image, particularly a colored image as implemented on their equipment, is outlined in the flowchart of figure 9. Typical hardware is indicated in Figure 10.

In general terms, a scene is electronically scanned from a film or other artwork to be printed, or recorded in some other digital process, and the data obtained is stored as pixel based color and intensity information. The pixels are generally aligned with the vertical and horizontal directions of movement of the scanner. The data is then processed into a standardised format such as known under the trade mark POSTSCRIPT and from there into up to four halftone screens which represent the primary colors and black as required. These screens are created from the pixel information by various raster image processor programs which calculate the dot shapes, dot frequencies and screen rotation angles. An operator normally has a range of dot patterns available through software installed on the equipment. The operator selects appropriate shapes, frequencies and angles in reaching an acceptable image during proofing. In traditional printing M.Z, PATENT OFFICE 7 - MAR 1996 received 272570 operations each screen is then output individually, to create up to four films or plates which are used to print multiple copies of the final black and white or colored image. This part of the process is known as imagesetting in the case of desktop publishing and as scanner output in the case of "high end" systems. In other computer based operations, such as ink jet of laser printing, it is possible to output a colored image directly.

Figures 1a, 1b and 1c show conventional round, square and "elliptical" dot patterns respectively, varying smoothly in size from about 10% print area density in light tones to about 90% area density in dark tones. Round dots have outwardly curved edges by definition whereas square or diamond shaped dots have flat edges between four points. The square patterns are invertible in that the printed areas in light tones have similar shapes to the non-printed areas in dark tones and vice versa.

Figure 2 demonstrates a more complicated dot pattern in which conventional round dots in light tones become square dots in middle tones with reversal of the round dots in dark tones. This is often referred to as the Euclidean dot pattern. The edge curvature of printed areas correspondingly changes from outward to flat to inward respectively. Moir6 effects do not arise in Figures 1 and 2 as only a single screen is present in each case. Dot gain is not apparent due to the magnification.

Figure 3 shows a reversed round dot pattern varying uniformly from light to dark tones. The dots In light tones have inwardly curved edges between cusps distributed on two planes of mirror symmetry. Their points could perhaps be flattened or rounded as desired. Each dot meets its four nearest neighbours simultaneously at around 22% area density. Otherwise the inwards curvature extends along substantially the full length of all printed areas. The non-printed v ■?„. paitr.r off-ice 7 - MAR m 272570 areas have corresponding outwardly curved edges and separate to appear as round dots in the dark tones. Overall the pattern is seen to be effectively a reversal or negative of the conventional round dot pattern in Figure 1 a throughout the entire range of tones, and in this respect may be considered a pattern of light 5 round dots on a dark printed background. By virtue of this reversal all print area edges are circles or circular arcs but could be other smooth curves as desired, or as required to link with the end points. Also the dots need not be fourfold symmetrical as shown. Rosette molr6 effects in coloured images can be reduced by printing dot patterns having inwardly curved edges as will be evident from later 10 figures in which two screens are superimposed. Tone jumps due to dot gain remain a problem however, as v/ith the conventional round dot pattern.

Figures 4a and 4b show a reversed variable oval dot pattern according to the invention. In this form the generally pin cushion shaped dots are elongated along a plane of mirror symmetry in light quarter to middle tones, so that nearest 15 neighbours meet first vertically along the direction of tone darkening and then second horizontally in the corresponding perpendicular direction. Each dot meets two opposing pairs in two distinct stages of around 22% and 36% area density. This pattern is seen to be effectively a reversal or negative of a round dot pattern in which the dots become oval in middle tones. Figure 4a shows a full range of 20 tone from 0% to 100% while Figure 4b shows more detail of the fourfold/twofold or alternatively the round/oval variation from about 15% to 39% area density. Rosette moir6 effects can be reduced as with the pattern of Figure 3, but now by controlling dot elongation the perceptibility of any tone jumps can also be reduced by splitting dot gain into two less obvious stages which in turn may be shifted 25 among a range of tones. -ecx-ss.-iam j* n.z, patent 272 570 Figures 5a, 5b and 5c show example variations of dot edges according to the invention superimposed on common centres. The diagonally lined squares facilitate comparison and measurement in production. The edges are not necessarily mathematical ellipses but may take any suitable oval or approximately similar form as can be generated by computer. Figure 5a shows the gentle round-oval-round variation of Figures 4a and 4b. Figures 5b and 5c show more severe distortions resulting in greater separation of the dot gain stages. These Figures are contour patterns which have been used in manual preparation of threshold matrices as described on pages 15 and 16 of the Seybold Report mentioned above. The process is also described in the appendix to this specification.

Figures 6a and 6b are an illustration of the reduction in rosette moir§ effect which can be achieved according to the present invention. Figure 6a shows rosettes created by superimposing two conventional round dot screens at 30°, as is common with cyan and magenta in coloured images. Figure 6b shows a smoother variation of similar tones created by superimposing two reversed variable oval dot screens at 90° according to the invention. Preparing colored images using all four possible screens is less straightforward.

It has been found that the primaiy color screens are apparently best placed at 45° angular separations, such as cyan 45°, magenta 135° and yellow 90°, with black placed at 0° or 90°. Further, that not every screen need be printed according to the invention. For example, at least cyan and magenta should use elongated dots of the reversed variable oval dot pattern, while yellow may use only the reversed round dot pattern. Black may only need the reversed round dot pattern, or may even give satisfactory images using the conventional round dot pattern itself. n.z. patent office 7- MAR 1996 r-l Cuivrrj 272 570 It has also been found that black dots are often not necessary in preparing a well defined colored image. For example, the primary color screens may be used alone at the angles and with the patterns mentioned above.

It is also generally required that the cyan and magenta screens should be offset from each other to avoid possible color shifts. The offset should be about half a cell parallel with either of the screen directions, for best results in view of the accidental offsets which often occur during printing. Yellow and black screens should also sometimes be offset to avoid color shifts. The nature of these offsets has yet to be fully explored but will be evident to a skilled reader working on a particular image.

In some images the possibility of color shifts and moir6 effects has been reduced with the 45° angles and reversed patterns mentioned above, by decreasing the black screen ruling (increasing the dot frequency) by a factor of about cosine 45° (about 0.71) relative to cyan and magenta. In a smaller number of cases the yellow screen ruling has been correspondingly increased by this factor. Again the nature of these adjustments to the spacings has yet to be fully explored, but will be evident to a skilled reader working on particular images.

Figures 7a to 7i show the dot patterns of a test which has been carried out to date according to the invention. Figures 7a to 7d represent individual screens of cyan 135°, magenta 45°, yellow 0° and black 0/90°, all at tone values where the printing and non-printing areas have linked with two or four of their nearest neighbours. Cyan and magenta use the reversed variable oval dot pattern. Yellow and black use simply the reversed round dot pattern. The black screen ruling has been reduced by a factor of about 0.71 relative to the others. Figures 7e to 7g show screens superimposed in pairs, namely cyan/yellow, magenta/yellow and cyan/magenta. In figures 7f and 7g the cyan and magenta N.Z. PATENT OFFICE 272570 have been offset by half a cell parallel to their screen directions, which are diagonal on the page as shown. The visible effect is somewhat unrealistic in that yellow normally has substantially less impact on the eye than the darker colors but here all colors must be shown equally in black. Figure 7h shows the cyan, magenta and yellow screens superimposed. A very slight moir6 effect is apparent primarily because the colors must be shown in black as mentioned. Figure 7i shows the black screen of figure 7d superimposed on figure 7h. Again there is a slight moir6 effect due to the overall black color representation whereas under normal printing circumstances the colors would have different impacts and the final image screen would be virtually moird free.

Figures 8a to 8i show an attempt to contrast with figures 7a to 7i, the conventional round dot patterns which would typically have been used. Figures 8a to 8d represent Individual screens of cyan 15°, magenta 45°, yellow 0° and black 75°. Figures 8e to 8g show these screens superimposed in pairs, namely cyan/yellow, magenta/yellow and cyan/magenta. Figure 8h shows the cyan, magenta and yellow screens superimposed. Figure 8i shows the black screen of figure 8d superimposed on figure 8h. Both large and small scale moird effects can be seen in these figures, although the effects are not so apparent as they normally would be in a complete colored image at normal screen rulings. The effects of dot gain are not at all apparent due to the artificial manner in which these figures must be presented.

Figure 10 is a highly schematic diagram of high end computer controlled apparatus which might be used by a human operator in implementing the process of Figure 9. The operator interfaces with the apparatus through a keyboard or other appropriate hardware and software 140. A recorded scene such as a photograph is passed under a scanner 110 to produce pixel data. Data is stored 272570 in memory 130 typically a hard disc. A computer processor 100 is then used to select and modify dot patterns in the pixel data. Halftone screens in primary colours or black are output through a printer/image setter 120. The screens are combined in preparation of a high quality halftone image. ) patent crrctlj ■; - mar i l "" RLCG-IVED __| 272570 - 15" APPENDIX This is a simple explanation of the process by which half-toning may take place on a computer, or computerised equipment such as an imagesetter. The 5 equipment is known technology, with the present invention relating to variations of dot shape and screen angles implemented on the equipment.

The fundamental operation is the application of a half toning pattern to a continuous tone original image to produce a binary (black or white) image in such 10 a way that the visual effect approximates the continuous tone onginal image. An Identical process is carried out for each primary colour or black screen. This process revolves around the threshold matrix: a grid of numbers or grey levels that specifies the size and shape of each printed dot desired for various levels of grey in the image to be printed.

The table below is one possible threshold matrix to implement a dot shape according to the invention on a 16x16 grid as would be used, for instance, for a 150 Ipi screen on a printer operating at 2400 dots per inch. 253 248 234 197 156 113 103 99 97 102 112 158 196 233 246 252 249 239 207 170 132 104 85 80 82 86 106 133 169 206 237 245 242 223 185 145 117 90 78 71 70 76 91 116 144 184 220 243 228 201 173 142 109 73 62 57 56 63 74 108 141 172 203 231 225 192 165 138 93 53 51 43 34 45 52 94 137 167 195 227 217 189 161 131 67 38 29 19 18 24 37 64 130 160 190 218 212 181 155 124 48 13 8 7 14 26 49 126 152 183 213 209 176 151 120 32 17 2 3 4 16 122 148 17 7 210 211 178 150 123 36 9 0 1 11 21 39 121 149 179 208 214 180 153 125 46 6 12 27 50 127 154 182 215 216 188 162 128 65 33 28 23 22 31 41 66 129 163 191 219 224 193 166 139 92 55 47 40 42 44 54 95 136 164 194 226 230 200 174 140 110 75 60 58 59 61 72 111 143 175 202 229 241 221 186 146 118 88 77 68 69 79 89 119 147 187 222 240 247 236 205 168 134 1( ftt ftd 107 135 171 204 238 244 254 250 235 198 157 1 ned1 •am r ob iqb0 114 159 199 232 251 255 7- MAR 1996 Deceived 272 57 0 " 16 " The interpretation of this table is that, starting from a completely black pattern, the first pixels to be whitened are In a roughly circular pattern in the middle. By the time 100 pixels have been whitened, the white area extends fully to the top and bottom of the pattern, but only hallway to the left and right sides (note the sudden jump firom values in the mid-Bo's to more than 120 as you proceed left or right from the middle in the middle rows).

The process of creating a threshold matrix such as the one above consists of two steps: deciding what shape dots you want, and how the shape should change at various dot sizes. This process will affect the overall look of the printed image, and the exact shape selected will determine various properties of the printed image, such as tone-jump and for multi-colour printing, moire patterns.

This is the subject addressed by the present invention. creating the threshold array in a standard fashion. This can be performed manually, or by a mathematical algorithm.

The manual process consists of overlaying a grid with "contour lines" such as those shown in Figures 5a, 5b, 5c which determine the desired dot shapes at various grey levels (10%, 20%, 30%, etc) and writing down the numbers 0,1,2,3... in the cells enclosed by each contour line in turn, until the entire table is filled in.

— N.Z. PATENT OFFICE 7 - MAR 1998 rlcnived 272570 -17 _ The mathematical process - which can be used for sufficiently simple shapes - consists of developing a mathematical function (called a spot function) that takes x and y coordinates within the halftone cell and returns a value indicating the "height" of that point. The heights so generated are then sorted by the computer and assigned the numbers 0, 1, 2, 3 ... as above.

Example mathematical functions include f(x,y) = x for a vertical line pattern, f(x,y) - y for a horizontal line pattern, and f(x,y) = /fx2 + ^; for a conventional circular dot screen.

A manual process was used in developing the present invention through the prototype stages described in the provisional and complete specifications.

Once you have your threshold matrix, the actual algorithm used to apply it is simply to consider each cell in a particular halftone screen in turn and compare the number written there against the desired grey level (also expressed as a number from 0 to 255, with 0 being black and 255 being white) in the image to be printed. If the number from the table is larger than the desired grey level then that pixel is printed as white, otherwise it is printed as black.

This process can be carried out in any of a number of ways: photographically, by arranging for light from the desired image to shine through a transparent sheet imprinted in the above pattern, and thence 272570 onto high-contrast film that will quite suddenly change from transparent to opaque at some fixed level of exposure to light. by a program on a computer workstation, with the resulting black and white result stored on a hard disk, or displayed on a TV monitor, or sent as a bitmap to a simple dot matrix, Inkjet, laser, or other printer. by a program built into a sophisticated computerised printer. Many printers have several halftone screen patterns built into their software. In order to be useful for the various dot shapes, it must be possible for the user to specify the pattern to be used for half toning.

The most common high-end output devices in the pre-press industry today are based on a computer programming language called PostScript, and we have concentrated our efforts on these patterns, though the invention^described in the specification apply equally well to other printing and display devices.

In printers that use Level 2 PostScript, it is possible for the user to directly specify the threshold matrix to be used to the printer, as in this example of one of the prototype shapes (this is the same table as previously shown): -19 . 272570 diet begin /HalftoneType 3 def /Width 16 def /Height 16 def /Thresholds 256 string def [253 248 249 239 242 223 228 201 225 192 217 189 212 181 209 176 211 178 214 180 216 188 224 193 230 200 241 221 247 236 254 250 234 197 156 207 170 132 185 145 117 173 142 109 165 138 93 161 131 67 155 124 48 151 120 32 150 123 36 153 125 46 162 128 65 166 139 92 174 140 110 186 146 118 205 168 134 235 198 157 113 103 99 104 85 80 90 78 71 73 62 57 53 51 43 38 29 19 25 13 8 17 5 2 20 9 0 30 15 6 33 28 23 55 47 40 75 60 58 88 77 68 105 87 81 115 101 98 97 102 82 86 70 76 56 63 34 45 18 24 7 3 1 14 4 11 12 22 31 42 44 59 61 69 79 83 84 96 100 112 158 196 106 133 169 91 116 144 74 108 141 94 137 64 130 49 126 35 122 39 121 50 127 66 129 95 136 72 111 143 89 119 147 107 135 171 114 159 199 52 37 26 16 21 27 41 54 233 246 206 237 184 220 172 203 167 195 160 190 152 183 148 177 149 179 154 182 163 191 164 194 175 202 187 222 204 238 232 251 0 1 255 {2 copy get Thresholds 3 end sethalftone 252 245 243 231 227 218 213 210 208 215 219 226 229 240 244 255] -1 roll put} for If the desired dot shape is simple enough to be reduced to a mathematical function, it is also possible to specify this directly to the PostScript printer. For instance, the standard circular dot f(x,y) = S(x* + y2) can be created as follows: 150 0 {dup mul exch dup mul add sqr This will create a 150 Ipi screen at an angle of zero degrees. . .tent office 7 - MAR 1996 f'.cceiveo

Claims (24)

WHAT WE CLAIM IS: -20- 27 2 5 7 0

1. A method of preparing halftone screens prior to printing a colored image, comprising: (a) receiving information representing color and tone variation in a previously recorded image; (b) processing the information to create halftone screens representing primary colors in the recorded image, with aach screen having a pattern of printing areas, by: (I) creating cyan and magenta screens to have respective patterns of printing areas with inwardly curved edges, and (ii) creating each primary color screen to have the respective pattern of printing areas at an angle of approximately 45°, 90° or 135° to the pattern of any other primary color screen; and (c) producing halftone screens representing the primary colors in a form appropriate for subsequent printing of the colored image.

2. A method according to claim 1 further comprising: creating the cyan and magenta screens having printing areas in light tones formed as dots which are increasingly elongated with darkening tones.

3. A method according to claim 1 further comprising: creating the cyan and magenta screens having non printing areas in dark tones formed as dots which are increasingly elongated with lightening tones. n.z. PATENT . ■ c 2 6 JUL 1996 .21. 27 2 5 7 0

4. A method according to claim 2 or 3 further comprising: joining the dots In two distinct stages as tone darkens or lightens respectively.

5 5. A method according to claim 1 further comprising: creating cyan and magenta screens which are relatively offset.

6. A method according to claim 1 further comprising: creating cyan and magenta screens having printing areas which 10 in light tones are inwardly curved on every edge.

7. A method according to claim 1 further comprising: creating every primary color screen having printing areas with inwardly curved edges. 15

8. A method according to claim 1 further comprising: creating a black screen having a pattern of printing areas with inwardly curved edges. 20

9. A method according to claim 1 further comprising: creating the cyan and magenta screens to have respective patterns at approximately 90° to each other.

10. A method according to claim 8 further comprising: 25 creating yellow and black screens to have respective patterns at approximately 90° to each other. n.z. PATENT 2 B JUL 1996 RElCDvf" 27 2 57 0 -22 -

11. A method according to claim 8 further comprising: creating the black screen to have a pattern at an angle which is approximately a multiple of 45° to any other screen. 5

12. A method according to claim 11 further comprising: creating the black screen with a screen ruling which is reduced by a factor of approximately 0.71 relative to the cyan and magenta screens.

13. A method of printing halftone screens to form a colored image, 10 comprising: (a) receiving information representing primary color halftone screens, wherein cyan and magenta screens have patterns of printing areas with inwardly curved edges; (b) orienting each primary color screen to place the respective 15 pattern of printing areas at an angle which is approximately 45°, 90° or 135° to the pattern of any other primary color screen; and (c) superimposing and printing the oriented primary color screens to form the colored image. 20

14. A method according to claim 13 further comprising: receiving information representing a black halftone screen; orienting the black halftone screen to place a respective pattern of printing areas at an angle which is approximately a multiple of 45° to the pattern of any other screen; and 25 printing the oriented black color screen with the primary color screens to form the colored image. N.Z. PATENT Of 2 6 JUL 1996 RCC'i'\ 272 570 -23-

15. A computer controlled apparatus for preparing halftone screens representing a colored image, comprising: (a) means for receiving information representing color and tone variation in a previously recorded image; (b) means for processing the information to create halftone screens representing primary colors in the recorded image, each screen having a pattern of printing areas; and (c) means for producing halftone screens representing primary colors in a form appropriate for subsequent printing of the colored image; wherein the means for processing creates cyan and magenta screens having respective patterns of printing areas with inwardly curved edges, and each primary color screen at an angle which places the respective pattern at approximately 45°, 90° or 135° to the pattern of any other primary color screen.

16. Apparatus according to claim 15 wherein the means for processing includes means for producing tone variation in the primary color halftone screens by creating printing areas which vary in elongation between light and dark tones.

17. Apparatus according to claim 15 wherein the means for processing includes means for producing tone variation in the primary color halftone screens by creating non-printing areas which vary in elongation between dark and light tones.

18. Apparatus according to claim 16 or 17 wherein the means for processing includes means for joining the printing or non-printing areas in two distinct stages as tone darkness or lightens respectively. M2L PATENT OFFICE 2 6 JUL 1996 272570 -24-

19. A computer controlled apparatus for preparing halftone screens representing a colored image, comprising: (a) means for scanning a previously recorded image to produce information representing color and tone variation in the recorded image; (b) means for processing the information to create halftone screens representing the primary colors and black tone variation in the image; and (c) means for producing the halftone screens in or on an output medium; wherein the means for processing includes means for creating cyan and magenta screens having respective patterns of printing areas with inwardly curved edges, and creates each primary color screen to place the respective pattern at an angle which is approximately a multiple of 45° to the pattern any other primary color screen.

20. A colored halftone image wherein tone variation in at least cyan and magenta is produced by patterns of printed areas having inwardly curved edges, and in which the primary color patterns are oriented at angles of approximately 45°, 90° or 135° to one another.

21. A process of preparing a halftone image wherein tone variation in cyan and magenta screens is produced by creating patterns of printing areas which have inwardly curved edges, and wherein the pattern of each primary color screen is placed at an angle of approximately 45°, 90° or 135° to any other primary color screen. 2 6 JUL 1996 272 570

22. A method according to any one of claims 1 to 14 substantially as herein described with respect to the accompanying drawings.

23. Apparatus according to any one of claims 15 to 19 and substantially as here described with respect to the accompanying drawings.

24. A colored halftone image substantially as herein described with respect to the accompanying drawings. DATED THIS ^ DAY OF 19 % A. J. PARK & SON AGENTS FOR THE APPLICANTS n.2. PATE NT Qrp,CE JUL 1998